Problem: Which of these numbers is prime? ${51,\ 67,\ 68,\ 69,\ 91}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 51 are 1, 3, 17, and 51. The factors of 67 are 1 and 67. The factors of 68 are 1, 2, 4, 17, 34, and 68. The factors of 69 are 1, 3, 23, and 69. The factors of 91 are 1, 7, 13, and 91. Thus, 67 is a prime number.